Many internal combustion engines, such as engines operating on the four-stroke principle, have intake and exhaust valves provided in the cylinder head of the engine. The intake valves open and close to selectively communicate the air intake passages of the engine with the combustion chambers of the engine. The exhaust valves open and close to selectively communicate the exhaust passages of the engine with the combustion chambers of the engine.
To open the valves, many engines are provided with one or more camshafts having one or more cams provided thereon. The rotation of the camshaft(s) causes the cam(s) to move the valves to an opened position. Metallic coil springs are usually provided to bias the valves toward a closed position.
Although metallic coil springs effectively bias the valves toward their closed positions for most engine operating conditions, at high engine speeds, the metallic coil springs have a tendency to resonate. When resonating, the metallic coil springs cause the valves to vacillate between their opened and closed positions, which, as would be understood, causes the intake and exhaust passages inside which the valves are connected to be opened when they should be closed. This results in a reduction of operating efficiency of the engine at high engine speeds.
One solution to this problem consists in replacing the metallic coil springs with air springs. An air spring typically consists of a cylinder having a piston therein. An air chamber is defined between the cylinder and the piston. The valve (intake or exhaust) is connected to the piston of the air spring. When the cam moves the valve to its opened position, the piston of the air spring moves with the valve, thus reducing the volume of the air chamber and as a result increasing the air pressure therein. When the cam no longer pushes down on the valve, the air pressure inside the air chamber causes the piston of the air spring to return to its initial position and to return the valve to its closed position.
Air springs do not resonate at high engine speeds the way metallic coil springs do. Also, for equivalent spring forces, air springs are lighter than metallic coil springs. Furthermore, air springs have progressive spring rates, which means that the spring force of an air spring varies depending on the position of the piston inside the cylinder of the air spring, which may also be advantageous for certain engines.
Although air springs offer many advantages over metallic coil springs, they also have some deficiencies that need to be addressed.
One of these deficiencies is that during operation, some of the air inside the air chamber of the air spring blows by the piston as the piston moves to reduce the volume of the air chamber. As a result, the air pressure inside the air spring is reduced, thus reducing the spring force of the air spring. This results in the valve not returning to its closed position as fast as it should, thus reducing the efficiency of the engine. In extreme cases, it is possible that the air pressure inside the air spring is insufficient to return the valve to its closed position. Since the valve remains in its opened position, the engine no longer operates properly, and the piston of the engine can come into contact with the valve, potentially damaging the valve.
One solution consists in providing a reservoir of pressurized air in fluid communication with the air springs that replenishes the air inside the air springs as it leaks out of the air springs. However, the pressurized air inside the reservoir is eventually depleted and the reservoir needs to be refilled or replaced. This can prove to be inconvenient for the users of the vehicle or device inside which the engine is provided.
Therefore, there is a need for a system for replenishing air inside an air spring used to bias a valve of an engine that does not require frequent replacement or refilling.
Another of the deficiencies associated with air springs is that even when the engine is not is use, air can leak out of the air springs. When the air pressure inside the air springs becomes too low, this causes the valves to move to their opened positions. When this occurs and the engine is started, the pistons of the engine can come into contact with the valves, potentially damaging the valves, and as a result preventing operation of the engine.
One possible solution consists in providing metallic coil springs having a relatively low spring constant in addition to the air springs. The metallic coil springs are strong enough to bias the valves towards their closed position even when the air pressure inside the air springs is no longer sufficient to do so on its own. However, these metallic coil springs do not provide enough biasing force to return the valves to their closed position fast enough while the engine is in operation. Although the addition of these metallic coil springs will prevent the pistons of the engine from coming into contact with the valves when the engine is started, they add weight and complexity to the air spring system. The additional metallic coil springs can also lead to some resonance as the speed of the engine increases.
Therefore, there is a need for a system for replenishing air inside an air spring used to bias a valve of an engine before the engine is started.